This invention relates to a stator unit mounted in a torque converter of a motor vehicle.
As shown in FIG. 17, a torque converter comprises an impeller 11 coupled to an output shaft of the engine, a turbine 13 coupled to an input shaft of the transmission and opposed to the impeller 11, and a stator 12 mounted through a one-way clutch 10 on a stator shaft fixed to a casing.
The stator 12 changes the flow direction of fluid that circulates between cup-shaped impeller blades 11b and turbine blades 13b when the fluid is returned from the turbine 13 to the impeller 11 at the radially inner portion of the blades 11b and 13b, thereby applying a turning force to the impeller 11 in the direction in which the impeller 11 is turning, and thus amplifying the torque transmitted between the impeller 11 and the turbine 13.
The one-way clutch 10 comprises an outer ring 21 joined to a stator hub 12a of the stator 12, an inner ring 22 joined to the stator shaft, a plurality of sprags 20 arranged circumferentially in an annular space defined between the inner and outer rings 22 and 21, and an annular retainer 23 retaining the sprags 20. The one-way clutch 10 permits rotation of the stator 12 in one direction only.
The outer ring 21 is heat-treated when formed. The outer ring 21 and the retainer 23 each have one axial end thereof supported on an end wall 12a′ of the stator hub 12a, and have their other axial ends pressed from outside by a presser plate 14. The outer ring 21 and the retainer 23 are supported by the stator hub 12a while being sandwiched between the end wall 12a′ and the presser plate 14.
Thrust bearings 1a and 1b are mounted between the stator 12 and the impeller 11 and between the stator 12 and the turbine 13, respectively, to bear axial loads applied to the stator 12. The thrust bearings 1a and 1b each comprise bearing rings, radially extending rollers, and a retainer retaining the bearing rings and the rollers.
A stator unit comprising the stator 12, one-way clutch 10, and thrust bearings 1a and 1b is mounted in a torque converter together with the impeller 11, the turbine 13, etc.
Among such stator units, there are known ones including side plates provided axially outwardly of the end wall of the stator hub and the presser plate, respectively, and snap rings each axially positioning one of the side plates, so that the side plates serve as the bearing rings of the respective thrust bearings (as disclosed in JP patent publication 2003-343690A).
Also, as shown in FIG. 18, there are known stator units in which a small-diameter step 17 is formed on the outer periphery of the outer ring 21 at one axial end thereof so as to be smaller in diameter than the remaining portion of the outer periphery of the outer ring 21. The bearing ring 16 of the thrust bearing 1b has its radially outer end portion 16a bent along the small-diameter step 17 so as to directly engage the outer ring 21 not through any intervening member. Also, the bearing ring 16 has its radially inner end portion 16b protruding radially inwardly of the outer periphery of the inner ring 22 so as to axially and directly oppose the end surface of the inner ring 22 not through any intervening member (as disclosed in JP patent publication 2005-214312A).
Some of such one-way clutches include sprags disposed between the inner and outer rings and configured to selectively engage and disengage from the inner and outer rings according to the relative position of the inner and outer rings, thereby selectively transmitting torque between the inner and outer rings.
The sprags of some of such one-way clutches are circumferentially spaced apart from each other, and the sprags of other such one-way clutches are kept in contact with the circumferentially adjacent sprags.
There is known a sprag type one-way clutch of the former type in which the numerous sprags are disposed annularly between the inner and outer rings, retained by a retainer, and biased by a spring called a garter spring in a direction in which the sprags engage, and which includes a pair of annular side plates each provided on one side of the sprags (one axial end of the clutch) (as disclosed in JP patent publication 59-73632A, FIG. 3).
There is known a sprag type one-way clutch of the latter type in which the numerous sprags are disposed annularly between the inner and outer rings so as to be sandwiched between a garter spring and a retainer, and biased by the garter spring in a direction in which the sprags engage, and which includes annular plates each disposed between one side of the sprags and the retainer (as disclosed in JP patent publication 58-163832A).
The stator unit disclosed in the abovementioned JP patent publication 2003-343690A has a large axial dimension because the bearing ring of one of the thrust bearings is located axially outwardly of the end wall of the stator hub.
The stator unit disclosed in the abovementioned JP patent publication 2005-214312A has a problem in that some of the parts forming the one-way clutch, such as the thrust bearings and sprags, could come off the clutch when the stator unit is mounted to the torque converter, because one of the bearing rings of the thrust bearings is fixed to the outer ring. Thus, a stator unit that can be handled more easily is desired.
Such a conventional one-way clutch used for a stator unit are assembled by inserting the numerous sprags one after another into the space between the inner and outer rings, and every time each sprag is inserted or after all of the sprags have been inserted, the sprags are brought into engagement with the garter spring one at a time. Thus, during assembly of the one-way clutch, their sprags tend to fall off.
In order to prevent separation of sprags during assembly of the one-way clutch, the garter spring could be mounted before inserting the sprags. But because the space into which the sprags are inserted is narrow, if the garter spring is inserted into this space before the sprags, the garter spring will interfere with the sprags, thus making it difficult to insert the sprags in position.
To solve these problems, there is known a one-way clutch as shown in FIGS. 15A and 15B which includes a cup-shaped outer ring 21 having a U-shaped section, wherein one of two axially opposed side walls 30a and 30b of the outer ring 21, i.e. the wall 30a, in which the garter spring 24 is inserted, is left unworked when the one-way clutch is assembled.
With the side wall 30a of the outer ring 21, in which the garter spring 24 is inserted, not bent radially inwardly, the sprags 20 and the garter spring 24, which is adapted to fit in recesses 25 of the sprags 20, are inserted as shown in FIG. 16A. Then, the side wall 30a is bent radially inwardly as shown in FIG. 16B. Finally, a flange 30d is formed at the radially inner end portion of the side wall 30a as shown in FIG. 16C.
In this arrangement, the side wall 30a of the outer ring 21 has to be bent radially inwardly with the sprags 20 and the garter spring 24 mounted in the outer ring 21. This is because if the side wall 30a of the outer ring 21 is completely bent radially inwardly relative to its peripheral wall 30c (as shown in FIG. 16B) beforehand, it is difficult to insert the sprags 20 and the garter spring 24 into the outer ring 21 because the gap w1 defined between the side wall 30a and protrusions 20d of the sprags 20 are small.
When the side wall 30a of the outer ring 21 is bent radially inwardly relative to its peripheral wall 30c with the sprags 20 and the garter spring 24 mounted in the outer ring 21, the sprags 20 and the garter spring 24 tend to move, thus making it difficult to accurately keep the bent position and shape of the side wall 30a. 
Also, such work is troublesome and thus tends to push up the manufacturing cost of the entire one-way clutch.